1. Field of the Invention
The invention relates generally to filter circuits with time variant characteristics controlled by switched capacitor networks. More particularly, the invention concerns improvements which minimize switching transients in such networks and filter circuits.
2. Description of the Prior Art
Filters in the integrated circuit arts are known which have time variant characteristics. One type of filter has a corner frequency which is variable from one frequency to another frequency, and is useful to remove noise from a signal. As used herein, xe2x80x9cnoisexe2x80x9d is defined as any undesired signal component caused due to parasitic coupling or switching of various nodes or components in a circuit. This type of filter is useful where the noise varies in frequency in a time dependent manner. When the characteristics of the noise are generally known, the time variant properties of the filter can be designed to best filter the noise while minimizing the adverse impact on the desired information in the signal.
One exemplary application of time variant filters is in improving the write-to-read settling time in the read channel of magnetic disk storage devices, or hard drives, for computers. This is an important application because it impacts directly with drive capacity. Write-to-read settling time is defined to be the time required for the read channel to settle and be able to read the next servo field after the write channel has stopped writing. The gap on the magnetic disk between the end of the write sector and the start of the servo field is wasted area and hence should be minimized.
The exact duration and nature of write-to-read disturbances are somewhat unpredictable. It depends on the assembly of the heads, sensors, flexible cable, as well as other noise contributing sources in the read channel, and hence varies between models of hard disk drives assembled by a manufacturer of such devices. When designing a preamplifier IC, the designer must model the interference sources, based on design data provided by the disk drive manufacturer as well as past experience (if any) with other models of that manufacturer, to design a suitable filter for interference suppression for the read channel. Once the noise characteristic is known, a circuit designer can design a filter which has for example, a time variant corner which generally tracks the frequency response of the noise signal desired to be eliminated.
A known technique for changing the characteristics of filters is to include a switched capacitor network in the filter. The corner frequency is changed by selectively switching different capacitances into circuit with the filter. The present inventors have recognized that the switched capacitor network is itself a source of noise.
An object of the invention, among others, is to reduce the noise generated by switched capacitor networks, and particularly as applied in time variant filters.
Generally speaking, according to one aspect of the invention, an integrated circuit includes a filter having an adjustable corner frequency. The filter includes at least one capacitive device switchable into circuit with the filter to adjust the corner frequency. An equalization circuit sets, prior to the capacitive device being switched into circuit with the filter, the terminals of the capacitive device to at least substantially the same electric potentials, and which electric potentials are at least substantially equal to the potentials to which these terminals are connected immediately after the capacitive device is switched into circuit with the filter circuit. This minimizes transient currents during switching of said capacitive device into the filter. This aspect of the invention is based on the recognition that if a voltage exists across the terminals of a capacitor when it is switched into the filter circuit, and if the potential of either of the terminals is changed as result of this switching, that it will cause a transient current to appear at the output of the filter. Since the purpose of a dynamically adjustable filter is typically to reduce noise in a system, the generation of additional noise by inclusion of the filter in the system should be avoided. Thus, the present invention provides a technique to drastically reduce one source of noise from switched capacitor networks and filters with which they are used.
Another aspect of the invention provides isolation for the switched capacitive devices during the precharging phase. Since the capacitive devices are selected to provide different predetermined corner frequencies, it is undesirable for these devices to influence the corner frequency. By providing isolation for the switched capacitive devices in the circuit, the effect on the corner frequency of the filter by each isolated device prior to such device being selectively switched into the circuit is substantially reduced. In one embodiment, the isolation is provided by serially coupled emitter-followers which form part of the equalization circuit. The emitter-followers reduce the reflection of an unswitched capacitive device by a factor of xcex22, where xcex2 is the well known transistor constant for the small signal forward gain.
According to yet another aspect of the invention, a controllable switch is coupled to a capacitive device of the network to switch the capacitive device into circuit with the filter. The switch has a switch capacitance and receives a control signal which induces a transient current in the filter circuit when the control signal is applied to the switch. The filter further includes a compensation circuit to generate a current opposite in sense and of substantially equal magnitude to the transient current, to thereby at least substantially cancel the transient current at the filter output. This aspect of the invention is based on the recognition by the present inventors that another source of unwanted noise in dynamically adjustable filter circuits is caused by the application of a control signal to switching devices in the filter, which in conjunction with a capacitance of the switch, such as the gate capacitance in a MOS transistor, induces a transient current which adversely affects filter performance.
According to an embodiment, the compensation circuit includes a capacitive compensation device, the capacitive compensation device receiving a compensation signal concurrently with the application of the control signal to the switch to generate the compensation current. The compensation device may be a MOS transistor having its source coupled to its drain, which increase the capacitance of the transistor, thereby allowing a smaller device. An advantage of this embodiment is that the compensation current is controllable through selection of the compensation signal applied to the gate of the MOS transistor.
Another aspect of the invention concerns methods of operating a switched capacitor network and a filter to implement the above-described techniques.
These and other object, features and advantages of the invention will become apparent with reference to the following detailed description and the drawings.